So far, a so-called vacuum thin-film formation technique such as a sputtering technique has generally be used to form a thin yet uniform oxide film between metal layers. When an oxidized film is formed by such a technique at a certain limited site in an ultrasmall electronic part of submillimeter order, dedicated masking is required, resulting in a drop of mass manufacturability and, hence, cost increases.
There is also available a so-called thick-film formation technique wherein a thick film is formed of a paste comprising an oxide dispersed in a vehicle by a screen printing process, a transfer process, a dipping process or the like. In this case, there is a certain limit to the formation of a uniform thick film of submicron order. Further, this technique involves elaborate process steps, because one metal layer is first formed, an oxide layer is then formed, and another metal layer is finally formed. Furthermore, it is often impossible to obtain any desired oxide film because some limitation is often placed on the firing atmosphere depending on the composition of the oxide.
It is here noted that an oxide layer has generally negative temperature characteristics. For some systems used in an environment having varying temperature conditions, however, it is desired to use therein a device having stable temperature characteristics or positive temperature characteristics.
By the way, a resistor chip is usually obtained by forming a resistor (film) on an insulator such as an alumina insulator. To this end, the resistor is first pasted, and the paste is then formed as by a screen printing process, followed by baking thereof to the alumina substrate. For such a resistor, a ruthenium oxide base resistor is primarily used optionally with tin oxide, tantalum oxide, etc. The resistor, comprising a mixture of high-resistance conductive particles, glass and a binder, is pasted, printed or otherwise formed at a predetermined position on an alumina substrate, and fired at a high temperature of 600.degree. C. or greater so that it can be baked to the substrate.
However, the resistance value is likely to vary with baking temperature. Especially upon baking for which atmospheric control is needed, the resistance value varies largely. In addition, a problem arises due to the incorporation of much glass component. That is, when baking is again carried out for the formation of other part, the glass is diffused under the influence of re-baking into the substrate, resulting in large resistance value variations.
When the terminal electrode portion of the resistor chip is plated, it is required to coat a protective film of resin or the like on the resistor formed by baking to provide protection against plating, because of the incorporation of much glass therein. Otherwise, plating erodes the glass or the like, and so causes large resistance value variations.
For most power supplies for recent electronic equipment, switching power supplies or DC-DC converters are used, and among capacitors used for these power supplies, there is a power supply bypass capacitor. For this power supply bypass capacitor, a multilayer ceramic capacitor of low capacity, and an aluminum or tantalum electrolytic capacitor of high capacity are used depending on the power capacity and switching frequency thereof, and a circuit parameter of a smoothing choke used in combination therewith. In this regard, the electrolytic capacitor functions well as a power supply bypass (smoothing) capacitor because large capacity is easily obtained. However, problems with this capacitor are that its size is large, and its low-temperature characteristics are poor with a risk of short-circuit accidents. In addition, not only do losses due to equivalent series resistance (ESR) occur steadily thanks to a relatively high internal impedance with the generation of heat, but also smoothing characteristics become worse due to poor frequency characteristics. As recently introduced innovative techniques enable the dielectric material or internal electrode of a multilayer ceramic capacitor to become thinner and thinner and have more and more layers, the electrostatic capacity of the multilayer ceramic capacitor comes close to the electrostatic capacity of an electrolytic capacitor. For this reason, various attempts have been made to substitute the electrolytic capacitor by the multilayer ceramic capacitor.
Of factors contributing to a smoothing effect in a power supply bypass capacitor, a ripple noise factor is of importance. To what degree the ripple noise is reduced is determined by the equivalent series resistance (ESR) of the capacitor. Here let .DELTA.Vr denote ripple voltage, .DELTA.i represent a current passing through a choke coil, and ESR stand for equivalent series resistance. Then, EQU .DELTA.Vr=.DELTA.i.times.ESR
This equation teaches that the ripple voltage is reduced by decreasing ESR. For a power supply bypass circuit, it is thus preferable to use a capacitor having low ESR. For this reason, some efforts are directed toward the application of a multilayer ceramic capacitor of low EST to a power supply circuit.
In a secondary circuit of a DC-DC converter, a switching power supply or the like, however, the ESR of a smoothing circuit has a large influence on the phase characteristics of a feedback loop, especially resulting in a problem that the ESR becomes extremely low. That is, when a multilayer ceramic capacitor of low ESR is used as a smoothing capacitor, a secondary smoothing circuit is equivalently constructed only of L and C components, causing oscillation to take place readily because the phase component present in the circuit is limited to only .+-.90.degree. and 0.degree. components or there is no margin of phase at all. In a power supply circuit using a three-terminal regulator, too, a similar phenomenon appears as an oscillation phenomenon at the time of load fluctuations.
For this reason, an electronic part comprising a multilayer ceramic capacitor of enhanced equivalent series resistance (ESR) has already been proposed. For instance, Japanese Patent No. 2578264 shows that a metal oxidized film is formed on the surface of an external electrode of a multilayer ceramic capacitor, so that the metal oxidized film can function as resistance to enhance ESR, and the resistance value can be controlled by the thickness of the oxidized film. With the method of fabricating this capacitor, however, it is difficult to control the oxidation of terminal electrodes. Even a slight increase in the degree of oxidation causes the interior of the electrodes to be oxidized, failing to achieve the desisred capacitor function. Even if only the terminal electrodes can somehow be oxidized, there is then inconvenience because the terminal electrodes are oxidized. That is, a plated film is formed by electroless plating. When plating is carried out according to this method, however, it is required to coat a ceramic material with a resin or the like for the purpose of preventing the ceramic material from being plated. This does not only make the fabrication process complex, but also renders the adhesion between the oxide and the plated film (Ni film) extremely low. As a consequence, the plated film peels off the oxide peels, failing to obtain the sufficient mechanical strength required for an electronic part. In other words, a lead wire is likely to be detached from the nickelled film after attached thereto.
As typically described in JP-A 59-225509, there is also known a resistor obtained by stacking a ruthenium oxide or other resistor paste on a multilayer ceramic capacitor, and co-firing them. When this resistor is immediately provided with terminal electrodes, however, an equivalent circuit is constructed of an RC or RLC parallel circuit, and so any series circuit cannot be obtained. When it is intended to obtain a series circuit, the shape of the terminal electrode becomes complex, and the fabrication process becomes complex, accordingly.